1. Technical Field
The present disclosure relates to a package for a MEMS (Micro-Electro-Mechanical System) sensor, in particular a capacitive microphone, and to the manufacturing process thereof.
2. Description of the Related Art
As is known, a MEMS sensor, e.g., an acoustic transducer such as a capacitive microphone, generally comprises a micromechanical sensing structure, designed to convert a mechanical stress (e.g., acoustic pressure waves) into an electrical quantity (for example, by exploiting variations of the electrical quantity in a capacitive structure of the MEMS sensor due to the acoustic pressure waves), and reading electronics, designed to carry out appropriate processing operations (including amplification and filtering) of the electrical quantity for supplying an electrical output signal (for example, a voltage).
Generally, an MEMS acoustic transducer is formed in a die including a structural layer of semiconductor material, for example silicon, accommodating a cavity. A membrane, or diaphragm, extends on top of the cavity; the membrane is flexible and, in use, undergoes deformation as a function of the pressure of the incident sound waves. A rigid plate (generally referred to as “back-plate”) extends at a distance from the diaphragm. The back-plate and the diaphragm thus form a movable electrode and a fixed electrode of a variable capacitor. The die further comprises contacts, used for biasing the membrane and the back-plate and for receiving an electrical signal resulting from the deformation of the membrane caused by the incident acoustic pressure waves.
The die implementing the acoustic transducer is enclosed in a package, accommodating also reading electronics associated thereto, generally provided as an ASIC in a respective die of semiconductor material.
A known package as above described is shown for clarity in FIG. 1.
FIG. 1 shows a package 1 including a cap 2 and a substrate 3. A first die 10, forming the MEMS sensing structure, and a second die 11, forming an ASIC integrating a reading electronics, are coupled side-by-side on the substrate 3. The first die 10 has a cavity 14 delimiting a diaphragm 15 (for simplicity, no back plate has been shown herein). Electrical connections 12 between the first and second dice 10, 11 and between the second die 11 and the substrate 3, are provided using a wire-bonding technique. Metallization layers and vias (not shown) are provided through the substrate 3 for routing the electrical signals towards the outside. Pads (in the case of an LGA—Land-Grid Array—package), or conductive spherical elements (in the case of a BGA—Ball-Grid Array—package), or similar connection elements, are moreover provided on the underside of the substrate 3 for soldering and electrical connection to an external printed circuit of a corresponding electronic device.
The cap 2 may be made of metal, or of a pre-molded plastic coated in the inside with a metallization layer, so as to prevent noise due to external electromagnetic signals (by providing a sort of Faraday cage). The cap 2 is generally attached to the substrate 12 by a conductive glue 17 so as to obtain also a ground connection towards the substrate 3. The cap 2 further has an opening 18 allowing acoustic pressure waves from the external environment to enter the package 1.
This known solution is susceptible of improvements. In particular, since the cap 2 is made by a molding technique, it requires specific and dedicated molding tools (comprising, for example, dies and punches), for each possible variation of dimensions and shapes, for example in case of variations of the silicon dimensions or in presence of different customer requirements. In addition, the pitch and layout of the molding and punching tools are not always compatible with the dimensions and configuration of the array of contacts.
Furthermore, this known solution has large dimensions for accommodating two dice side-by-side and arranging the cap, and in general it does not offer the designer a sufficient design freedom in sizing the front and back chambers of the acoustic transducer.
EP-1 755 360A discloses a package wherein the metal cap is secured to the substrate by welding rather than using a conductive epoxy.
US 2008/0063232 discloses a method of enclosing a silicon microphone in a plastic molded cap on which a metal layer has been deposited.
U.S. Pat. No. 7,166,910 in FIGS. 6-10 describes and shows a package layout with a MEMS transducer mounted on the top side of the package.
PCT/EP2010/070608, filed on 29 Dec. 2010 in the name of the same Applicant, discloses a package including a substrate carrying two dice including a MEMS chip and an ASIC; a wall structure, formed from a board and attached to the substrate to define a chamber accommodating the dice; and a cap layer upwardly closing the cavity. The dice are directly connected to connection elements formed on the face of the wall structure looking toward the cap layer.
All the above solutions may be improved in order to better exploit the available space and reduce the general bulk of the package for a given dimensions of the dice. In addition, a higher design freedom in sizing the back volume, on the back of the MEMS chip, is desirable.